1. Field of the Invention
The invention relates to a process for selectively separating iron from other metal ions, in particular ions present in certain oxidation catalysts.
It also relates to a process for recycling catalysts in the reaction for the oxidation of alcohols and optionally ketones to carboxylic acids and more particularly the oxidation of cyclic alcohols and cyclic ketones to dicarboxylic acids, such as the oxidation of cyclohexanol and/or cyclohexanone to adipic acid.
2. Description of the Related Art
Thus, it is known to manufacture adipic acid by nitric oxidation of a mixture of cyclohexanol and cyclohexanone. This oxidation is generally carried out in the presence of a catalyst comprising vanadium and copper.
The solution recovered after separation of the dicarboxylic acids and in particular of adipic acid is treated in order to allow the catalyst to be recycled in the oxidation reaction.
Several processes for the treatment of this solution have been provided. For example, the metals present in the solution can be extracted by treatment with ion-exchange resins. The solution, purified of metals, comprises the byproducts from the synthesis of adipic acid, namely glutaric and succinic acids. Such a process is disclosed, for example, in U.S. Pat. No. 3,965,164.
However, this process does not allow iron ions, originating in particular from the corrosion of the plants, to be removed. Thus, the catalyst is enriched in iron at each recycling cycle. This enriching can decrease the efficiency of the catalyst or can also contaminate the adipic acid manufactured.
Several processes have been provided for at least partially removing the iron without loss of vanadium and of copper.
Other processes consist in selectively and differentially eluting the vanadium and the copper from the ion-exchange resin by using nitric acid solutions at a more or less high concentration or elution solutions comprising acids other than nitric acid or phosphonic acid (SU 690,320, U.S. Pat. No. 3,554,692).
However, these processes do not make possible selective removal of the iron without significant loss of vanadium or copper, in view of the ratio of very low concentration of iron with respect to the other two metal ions.
To overcome these disadvantages, European Patent Application No. 0,761,636 provides a process which consists in treating the eluate comprising the iron, copper and vanadium ions with a second ion-exchange resin comprising aminophosphoric groups.
Although improving the prior processes, this process does not make it possible to remove most of the iron ions without loss, in particular, of vanadium ions, which is highly prejudicial to the oxidation process. This is because these resins can have a satisfactory selectivity for iron with respect to the vanadium only under very acidic pH conditions, of markedly less than 1, that is to say when the metal compounds are present in a concentrated nitric acid solution, for example.